Yeehaaa
here's a dummy way with a tetrahedron
fudge your model with a face on x,y plane and one edge on the x axis - you hope. this face must be filled
draw a flat rectangle joining this edge at both end and extend along the flat plane y. It should fill by itself. the line in between can be removed and it should all fill - if not you screwed up placing your model
I think this is fundamental to any modelling (would you start a 2d drg without an origin, scale and xy directions
coplanarity.jpg